<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        * {
            margin: 0;
            padding: 0;
        }

        ul {
            list-style: none;
        }

        .wrap {
            width: 330px;
            height: 30px;
            margin: 100px auto 0;
            padding-left: 10px;
            background-image: url(imgs/bg.jpg);
        }

        .wrap li {
            background-image: url(imgs/libg.jpg);
        }

        .wrap > ul > li {
            float: left;
            margin-right: 10px;
            position: relative;
        }

        .wrap a {
            display: block;
            height: 30px;
            width: 100px;
            text-decoration: none;
            color: #000;
            line-height: 30px;
            text-align: center;
        }

        .wrap li ul {
            position: absolute;
            top: 30px;
            display: none;
        }
    </style>
    <script src="jquery-3.4.1.js"></script>
    <script>
        $(function () {

            //mouseover：鼠标经过事件
            //mouseout:鼠标离开事件

            //mouseenter:鼠标进入事件
            //mouseleave：鼠标离开事件

            var $li = $(".wrap>ul>li");

            //给li注册鼠标经过事件，让自己的ul显示出来
            $li.mouseenter(function () {
                //找到所有的儿子，并且还得是ul
                $(this).children("ul").stop().slideDown();
            });

            $li.mouseleave(function () {
                $(this).children("ul").stop().slideUp();
            });


        });
    </script>
</head>
<body>
<div class="wrap">
    <ul>
        <li>
            <a href="javascript:void(0);">一级菜单1</a>
            <ul class="ul">
                <li><a href="javascript:void(0);">二级菜单11</a></li>
                <li><a href="javascript:void(0);">二级菜单12</a></li>
                <li><a href="javascript:void(0);">二级菜单13</a></li>
            </ul>
        </li>
        <li>
            <a href="javascript:void(0);">一级菜单2</a>
            <ul>
                <li><a href="javascript:void(0);">二级菜单21</a></li>
                <li><a href="javascript:void(0);">二级菜单22</a></li>
                <li><a href="javascript:void(0);">二级菜单23</a></li>
            </ul>
        </li>
        <li>
            <a href="javascript:void(0);">一级菜单3</a>
            <ul>
                <li><a href="javascript:void(0);">二级菜单31</a></li>
                <li><a href="javascript:void(0);">二级菜单32</a></li>
                <li><a href="javascript:void(0);">二级菜单33</a></li>
            </ul>
        </li>
    </ul>
</div>

</body>
</html>